Catalysts obtained by depositing particles of catalytic platinum or other catalytic precious metals on the surface of an appropriate carrier (e.g., a carbon carrier) have been widely used for applications such as exhaust emission cleaning or as an electrode in a fuel cell. For example, as the electrode catalyst of a polymer electrolyte-based fuel cell (also called PEFC: polymer electrolyte fuel cell), have been used precious metal-carrying catalysts wherein particles of a catalytic precious metal (typically, a platinum group element) are supported on a carbon carrier.
One of the requirements for development of a precious metal-carrying catalyst is reduction of the manufacturing cost. That is, to reduce the usage (carried amount) of a precious metal such as platinum, which is a factor contributing to a high cost, without lowering the catalytic performance. One of the means considered to achieve such an objective is to refine precious metal particles, that is, in other words, to employ nano-pulverization where the average particle diameter is reduced to an order of nm (typically, to about 1 nm to 100 nm average particle diameter based on electron microscopy (e.g., TEM) analysis). By the nano-pulverization, the exposed surface area in the precious metal particles is increased and the amount of the precious metal (platinum, etc.,) used per unit mass can be reduced.
Excessive nano-pulverization, however, may result in a decrease in the catalytic activity and thus is not preferred in view of maintaining the performance as the catalyst. For instance, with nano-pulverized platinum (Pt), when the particle diameter becomes below about 2.5 nm, the oxygen reduction activity per particle is likely to significantly decrease. Therefore, nano-pulverization seems to be limited to around 3 nm in view of maintaining the catalytic activity.
Alternatively, use of metal particles having a so-called core-shell structure where the catalytically non-active particle core is formed of a metal other than an expensive precious metal (i.e., a metal belonging to the non-precious metal group) has been also considered. For example, Patent Document 1 discloses a precious metal-containing catalyst in which a precious metal such as platinum is deposited on the surfaces of core particles formed of cobalt, iron, nickel or tungsten in order to reduce the usage of the precious metal element. The other related technical documents of this kind include Patent Documents 2 to 4 though the arts described in these do not directly contribute to achievement of the aforementioned objective.